1. Field of the Invention
The present invention relates to a wire electric discharge machining method of machining the electrically conductive workpiece by electric discharge. More particularly, the present invention relates to a wire electric discharge machining method of machining the workpiece with different thickness into a required shape with first cut and second cut.
2. Description of the Related Art
The wire electric discharge machining method is capable of removing material from the workpiece by repeatedly generating electric discharge across a work gap formed between the workpiece and a running wire electrode. The work gap is immersed in dielectric fluid such as deionized water. The electric discharge is generated by application of a series of controlled power pulses to the work gap. As illustrated in FIG. 1, the wire electrode E is vertically aligned between a pair of wire guides WG and moved relative to the workpiece W within a horizontal XY plane. During machining, a voltage across the work gap (“gap voltage”) is fed back to an NC device for CNC wire electric discharge machine. The NC device controls feed rate of wire electrode so that the mean gap voltage is maintained at a servo reference voltage. As a mean gap voltage is considered to be proportional to the size or distance of the work gap, the servo reference voltage is set in accordance with an optimum size of the work gap. A power pulse of larger electrical energy increases material removal rate while it decreases roughness of machined surface and shape accuracy. In general, in order to balance material removal rate, surface roughness and shape accuracy, wire electric discharge machining is divided into a number of machining steps. In preparation for machining, a wire path and a set of machining conditions are determined for each machining step. Firstly, the wire electrode is moved on a first wire path, and the workpiece is cut to a rough required shape with a large electrical energy at high speed. Such rough machining is called first cut. At the time first cut is completed, surplus material that must be removed remains on the cut surface, and the cut surface does not have the required roughness. Next, the cut surface is finished at a required shape accuracy using a small electrical energy. Such finishing includes several machining steps and is collectively called second cut or skim cut. During second cut, the wire electrode is made to move on wire paths so that the size of the work gap becomes smaller. In this manner, the surplus material is gradually removed to obtain the required shape accuracy, and the roughness of the cut surface is gradually reduced to a required value. Normally, a smaller electrical energy is supplied to a work gap of smaller size for each subsequent machining step.
FIG. 1 illustrates the workpiece W with different thickness being machined in first cut. Change in thickness of the workpiece W causes change in machining area which in turn results in undesirable change in size of the work gap. Japanese publication of examined application No. 63-025889 discloses a method in which change in thickness of the workpiece is detected based on a feedback of a mean gap voltage or feed rate and machining conditions are changed in accordance with the detected thickness of the workpiece. However, as removal material in second cut is much smaller than that in first cut, it is difficult to accurately detect change in thickness of the workpiece based on a feedback of a mean gap voltage or feed rate. In second cut, for example, if a wire electrode E reaches a position Q2 on a second wire path P2 and thickness of the workpiece W is decreased from t1 to t2, the size of the work gap is undesirably increased, as illustrated in FIG. 2.